The present invention generally relates to an atrial defibrillator which applies cardioverting electrical energy to the atria of a human heart when activity of the heart satisfies predetermined criteria. The present invention more particularly relates to such an atrial defibrillator which further selectively stores heart activity data related to the failure of the heart activity to satisfy the predetermined criteria after detection of an atrial fibrillation episode. The selectively stored data, herein referred to as "default data", includes data relating to the failure to confirm the initial detection of atrial fibrillation, failure to identify required heart activity conditions to enable an attempted cardioversion of the atria, or an inability to redirect atrial fibrillation or confirm reelection of atrial fibrillation after an attempted cardioversion of the atria.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes. Such strokes are thought to be caused by blood clots formed in areas of stagnant blood flow resulting from prolonged episodes of atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness due to decreased cardiac output.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide relief to patients suffering from occurrences of atrial fibrillation. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality. Two such proposed defibrillators, although represented as being implantable, required human interaction for cardioverting or defibrillating the heart (along with requiring a patient to recognize the symptoms of atrial fibrillation). One such defibrillator required the patient to visit a physician to activate the defibrillator. The other defibrillator required the patient to activate the defibrillator from external to the patient's skin with a magnet.
An improved atrial defibrillator is fully disclosed in U.S. Pat. No. 5,282,837, entitled IMPROVED ATRIAL DEFIBRILLATOR AND METHOD, which issued on Feb. 1, 1994 in the names of John M. Adams and Clifton A. Alferness. This patent is assigned to the assignee of the present invention and is incorporated herein by reference.
The atrial defibrillator of the above-referenced patent provides automatic operation. It senses activity of the heart and firstly determines if the heart is in atrial fibrillation by examining the heart ventricular rate, the ventricular rate variability, and the atrial activity. The ventricular rate and variability are used to predict the probability of atrial fibrillation when the rate and variability exceed a limit. The atrial activity is examined to determine with greater certainty if atrial fibrillation is present. When all of the atrial fibrillation detection criteria are satisfied, the atrial defibrillator cardioverts the atria of the heart.
The atrial defibrillator of the above-referenced patent includes further features and advantages. For example, it provides R wave detection of increased reliability for synchronizing the delivery of the cardioverting electrical energy to the atria with an R wave of the heart. This assists in avoiding the T wave vulnerable period of the heart when applying the cardioverting electrical energy to the heart. Further, as another feature, a lead system having electrodes in and near the heart reduces the amount of cardioverting electrical energy required to cardiovert the atria of the heart. This not only reduces energy consumption to prolong the useful life of the defibrillator, but, more importantly, reduces the potential discomfort to the patient during cardioversion.
Further improvements in implantable automatic atrial defibrillators are described in U.S. Pat. No. 5,207,219, which issued on May 4, 1993, for ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING INTERVAL TIMING PRIOR TO CARDIOVERSION, and which is also assigned to the assignee of the present invention and incorporated herein by reference. The atrial defibrillator there disclosed provides an answer to the observation that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high or variable cardiac rates, the R wave of a cardiac cycle may become closely spaced from the T wave of the immediately preceding cardiac cycle. This creates a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with the R wave close to the preceding T wave. In order to prevent cardioversion of the atria during an R on T condition, the atrial defibrillator described in U.S. Pat. No. 5,207,219 detects for a cardiac interval longer than a minimum interval prior to delivering the cardioverting electrical energy to the atria. This assures that the cardioverting electrical energy is not delivered during an R on T condition.
As can be seen from the foregoing, there is a complex criteria which the heart activity must satisfy for the automatic detection and cardioversion of atrial fibrillation. Such criteria may relate to both ventricular and atrial activity of the heart to detect fibrillation of the atria. The criteria may further relate to cardiac intervals immediately prior to cardioversion and the successful detection of R waves to assure that the application of the cardioverting electrical energy is synchronized with an R wave and avoids a T wave. The criteria may also relate to the quality of the cardiac signals or data derived therefrom as a prerequisite to evaluating the signals or data for detecting atrial fibrillation and applying cardioverting energy.
In addition to the foregoing, defibrillators have been developed which are capable of storing information relating to the successful detection and cardioversion of fibrillation. Generally, the stored information takes the form of digital samples representative of selected electrograms related to the detection of a fibrillation episode and the successful cardioversion of the detected fibrillation episode.
One such defibrillator is described in copending application Ser. No. 08/264,319, filed Jun. 23, 1994, in the names of Barry M. Yomtov and David P. Finch, for SELECTIVE DATA STORAGE FOR AN AUTOMATIC IMPLANTABLE ATRIAL DEFIBRILLATOR, which application is assigned to the assignee of the present invention and incorporated herein by reference. The defibrillator described in that application is an atrial defibrillator which stores, in memory, electrogram data related to the activity of the heart occurring during a discrete time period prior to detection of atrial fibrillation and electrogram data associated with the activity of the heart occurring during a second discrete time period commencing before cardioversion of the heart and extending continuously until after cardioversion of the heart. Once stored, this data may be transmitted through a telemetry link to an external receiver for display or chart recording to facilitate later confirmation of successful detection and cardioversion.
While such confirming data is of great importance to the cardiologist in monitoring patients, other information, not contemplated by the prior art to be stored, would also be of importance if made available. For example, data which may reveal the cause of a failure to treat a fibrillation episode once it is detected would also have utility. Such data would be especially helpful where a complex criteria must be satisfied to detect fibrillation, confirm such detection, and then apply cardioverting energy. Failure to treat a fibrillation episode could be caused by the failure to satisfy one or more different aspects of a complex criteria. Data relating to the failure to satisfy that criteria could facilitate corrective adjustment of programmable parameters of an implanted defibrillator to enable successful operation of such a device and the provision of appropriate therapy when required. It could also assist in revealing other operational problems, such as heart activity sensing difficulties resulting from sensing electrode migration or attempted operation in an environment having high electromagnetic interference.